Polymer electrolyte fuel cells are conventionally well known. Polymer electrolyte fuel cells include an anode and a cathode which consist of catalyst reaction layers and gas diffusion layers respectively and are arranged so as to hold a polymer electrolyte membrane therebetween. A hydrogen-containing fuel gas and an oxygen-containing oxidizing gas, such as air, (the fuel gas and oxidizing gas are hereinafter generically called as “reaction gases”) are supplied to the anode and the cathode. In the anode, electrons are released from the hydrogen atoms contained in the fuel gas through an electrode reaction thereby generating hydrogen ions and these electrons reach the cathode by way of an external circuit (electric load). Meanwhile, the hydrogen ions reach the cathode after passing through the polymer electrolyte membrane. In the cathode, the hydrogen ions, the electrons and the oxygen contained in the oxidizing gas combine, so that water is generated. In the course of this reaction, electric power and heat are generated simultaneously.
The known fuel cell as explained herein has a “membrane electrode assembly (MEA)”. The MEA is formed such that: a catalyst reaction layer containing, as a chief component, carbon powder that carries a metal having catalytic ability such as platinum is formed on both surfaces of a polymer electrolyte membrane that selectively transports hydrogen ions, and then a gas diffusion layer having fuel gas permeability and electron conductivity is formed on the outer surface of each catalyst reaction layer.
As illustrated in FIG. 16, the known fuel cell is provided with gaskets 40 that contact the peripheral region of the polymer electron membrane 7 so as to enclose the gas diffusion layers 5 and the catalyst layers 6 so that leakage of the supplied fuel gas and oxidizing gas to the outside and mixing of these gases are prevented. Further, this known fuel cell has electrically-conductive separators 41 that electrically serially connect adjacent MEAs 4 to each other. The MEA 4, the gaskets 40 and the separators 41 constitute a “cell”.
In the area of each separator 41 where the separator 41 contacts its associated gas diffusion layer 5 of the MEA 4, a gas flow path 3 is provided for supplying the reaction gas to the anode and the cathode and carrying generated gas and redundant gas away. Although the gas flow path 3 may be provided separately from the separator 41, the gas flow path 3 is generally defined by a groove formed on a surface of the separator 41 and the gas diffusion layer 5.
In the known fuel cell, since the mechanical strength of the polymer electrolyte membrane 7 is weak, a technique for reinforcing the peripheral region of the polymer electrolyte membrane 7 with a protective layer 10 as illustrated in FIG. 17 is employed (see Patent Documents 1, 2).
In the fuel cell described above, the MEA 4 needs to be uniformly pressurized by the separators 41. Non-uniform pressurization of the MEA 4 by the separators 41 causes such a phenomenon that some areas are strongly pressurized by the separators 41 whereas other areas are weakly pressurized.
In the areas of the MEA 4 strongly pressurized by the separators 41, local pressure concentration causes pin holes in the solid electrolyte membrane 7, because the solid electrolyte membrane 7 is poor in mechanical strength as discussed above.
In the areas of the MEA 4 weakly pressurized by the separators 41, the gas diffusion layer 5 and the separator 41 do not satisfactorily contact each other and therefore the contact area of the gas diffusion layer 5 and the separator 41 decreases as a whole with a drop in the voltage of generated power.
In addition, a gap is sometimes created between the separator 41 and the gas diffusion layer 5 in the areas of the MEA 4 weakly pressurized by the separators 41. The reaction gas supplied from a reaction gas feed manifold hole 2a is likely to pass through the gap (in the area indicated by B in FIG. 18) while traveling from the manifold hole 2a to a reaction gas discharge manifold hole 2b, so that the supplied reaction gas falls short in areas (indicated by A in FIG. 18) of the gas flow path 3. In the areas where the reaction gas is short, the polarization of the electrode reaction increases resulting in degradation of the performance of the fuel cell.
The general solid polymer electrolyte membrane 7 exhibits high ion conductivity in a wet condition and therefore, the reaction gases supplied to the gas flow paths 3 formed in the separators 41 are in a humidified condition. Further, the water produced by the electrode reaction makes the internal environment of the gas flow paths 3 more liable to generation of dew condensation water. To prevent the gas flow paths 3 from being clogged by the dew condensation water, the gas flow paths 3 of the conventional fuel cell are designed to be supplied with the reaction gas under enough pressure to remove the dew condensation water. However, in the areas of the gas flow paths 3 supplied with insufficient amounts of reaction gas, the capability of removing dew condensation water and, in consequence, the performance of the fuel cell decrease.
As an attempt to uniformly pressurize the gas diffusion layers 5 by the separators 41, the following measure is commonly taken in the fuel cells such as described above: the surface height of each separators 41 in contact with its associated gas diffusion layer 5 is made uniform, thereby making the contact surface of the separator 41 and gas diffusion layer 5 uniform.
Japanese Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-242897 Japanese Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-47230